1. Field of the Invention
The present invention relates to a technology that is well adaptable to an electric storage device including plural negative electrodes.
2. Description of the Related Arts
High energy density and high output density are required for an electric storage device that is mounted to an electric vehicle, a hybrid vehicle, or the like. Therefore, a lithium ion secondary battery, an electric double layer capacitor, etc. have been nominated as a candidate for the electric storage device. However, the lithium ion secondary battery has a high energy density, but low output density. The electric double layer capacitor has a high output density, but low energy density.
In view of this, there has been proposed an electric storage device called a hybrid capacitor in which the electric storage principles of the lithium ion secondary battery and those of the electric double layer capacitor are combined. The hybrid capacitor employs an activated carbon, which is used for an electric double layer capacitor, for a positive electrode so as to accumulate charges by utilizing the electric double layer in the positive electrode, and employs a carbon material, which is used for a lithium ion secondary battery, for a negative electrode, and lithium ions are doped into the carbon material of the negative electrode so as to accumulate charges. The application of the electric storage mechanism described above makes it possible to enhance the output density and the energy density. However, a further improvement in the output density and the energy density has been demanded in order to use the electric storage mechanism described above for a vehicle power source.
Methods for enhancing both an output density and an energy density of a battery include the one in which an internal resistance is decreased by coating an electrode mixture material to be thin, and the one in which a battery having a high energy density and a capacitor having a high output density are connected in parallel. However, in the former method, the electrode mixture material is coated to be thin, which entails a reduction in the energy density of the electric storage device, or which makes the assembly difficult to thereby increase cost of the electric storage device. In the latter method, the battery and the capacitor are combined, which entails increased cost of the electric storage device due to a complicated control circuit.
In order to solve these problems, there has been proposed an electric storage device in which positive-electrode current collectors of a lithium ion secondary battery and those of an electric double layer capacitor are connected to each other, and negative-electrode current collectors of the lithium ion secondary battery and those of the electric double layer capacitor are connected to each other (e.g., see JP-A-2001-351688). Further, an electric storage device has been proposed in which a mixture material including an active carbon or the like and a mixture material including a lithium cobalt oxide or the like are double-layer coated on a single current collector (e.g., see JP-A-2000-36325 and JP-A-2005-203131), or an electric storage device has been proposed in which a mixture material having mixed therein an active carbon and a lithium cobalt oxide is coated on a single current collector (e.g., see International Publication WO2002/41420).
However, in the electric storage device disclosed in JP-A-2001-351688, it is difficult to cancel the deviation in the potential between the electrodes that are connected to each other. Therefore, over discharge or overcharge of the lithium ion secondary battery or the electric double layer capacitor can occur. The occurrence of the over discharge or overcharge described above causes the deterioration in the durability of the electric storage device. In the electric storage devices disclosed in JP-A-2000-36325, JP-A-2005-203131, and International Publication WO2002/41420, it is difficult to ensure the output density by sufficiently reducing the internal resistance, since the device has a structure in which the active carbon and the lithium cobalt oxide are mixed or a structure in which the active carbon and the lithium cobalt oxide are laminated. Further, the electric storage devices have a structure in which the lithium cobalt oxide is in contact with the active carbon. Therefore, the effect caused by the deteriorated lithium cobalt oxide also affects the active carbon, which deteriorates the durability of the electric storage device.